Having an overactive thyroid gland is linked with an
increased risk for atrial fibrillation (a type of abnormal
heart rhythm), but neither an over- or under active thyroid
gland is associated with a higher risk for other
cardiovascular problems or increased risk of death,
according to a study in the March 1 issue of JAMA.

Thyroid hormone excess and deficiency are common, and can be
readily diagnosed and treated. Previous studies have suggested that
abnormal levels of thyroid stimulating hormone (TSH) may represent a
cardiac risk factor. Cardiovascular diseases (CVD) are the most
common cause of death in the U.S. Even mildly altered thyroid status
reportedly affects serum cholesterol levels, heart rhythm and rate,
ventricular function, risk of coronary artery disease, and
cardiovascular death. However, the relationship between abnormal
thyroid function and cardiovascular outcomes remains unclear,
according to background information in the article.

Anne R. Cappola, M.D., Sc.M., of the University of Pennsylvania
School of Medicine, Philadelphia, and colleagues tested the
hypothesis that abnormal thyroid status is associated with increased
cardiovascular risk and death in individuals with unrecognized
thyroid dysfunction. The study included 3,233 U.S. individuals aged
65 years or older who had their serum thyroid-stimulating hormone
levels measured when enrolled in 1989-1990. The cardiovascular
health of the patients, who were not taking thyroid medication, was
assessed through June 2002.

The researchers found that 82 percent of participants had normal
thyroid function, 15 percent had subclinical (before symptoms)
hypothyroidism (an underactive thyroid gland), 1.6 percent had
symptomatic hypothyroidism, and 1.5 percent had subclinical
hyperthyroidism (an overactive thyroid gland). After exclusion of
those who had atrial fibrillation at the start of the study,
individuals with subclinical hyperthyroidism had nearly twice the
incidence of developing atrial fibrillation compared with those with
normal thyroid function. No differences were seen between the
subclinical hyperthyroidism group and the normal thyroid function
group for the occurrence of coronary heart disease, cerebrovascular
disease, cardiovascular death, or all-cause death. Likewise, there
were no differences between the subclinical hypothyroidism or
symptomatic hypothyroidism groups and the normal thyroid function
group for cardiovascular outcomes or death.

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"Our analyses do not support screening older individuals solely to
prevent atrial fibrillation, with an estimated number needed to
screen of 2,500 older individuals to find 1 case of atrial
fibrillation associated with subclinical hyperthyroidism. Our
findings suggest that if endogenous [originating from within the
thyroid] subclinical hyperthyroidism is detected, older individuals
may benefit from treatment to prevent atrial fibrillation," the
authors write. "Our analyses do not support screening older
individuals for thyroid disease to prevent CVD, and, although our
data are observational, they do not support treatment of individuals
with subclinical hypothyroidism to prevent cardiovascular events."